The Earth's limited supply of natural resources will only be
able to sustain 2 billion humans by 2100, bad news for a
world that already feeds 5.9 billion.
The optimum human population, or carrying capacity, for
the U.S. is projected to be 200 million, which is millions fewer than
the current population.
Disappearing stocks of fertile land, fresh water, fossil fuel
energy, plants and animal life will control human population
the old-fashioned way-through starvation and disease-if we
cannot reduce our numbers voluntarily.

A SMALL WORLD AFTER ALL

The Earth and its resources may be too small for all of
us to share. Even if we learn how to make the most of a
limited supply of land, energy, water, and biota, Cornell
ecologists have calculated that by the year 2100, the planet
will be able to provide for only 2 billion humans-almost 4
billion less than today's world population-with a modest but
comfortable standard of living. Only 200 million humans can be
sustained by the natural resources of the United States,
making the current population 33 percent over its eco-budget.

"If we refuse to reduce our numbers ourselves," warns
David Pimentel, PhD, professor of entomology and
agricultural sciences, "nature will find much less pleasant
ways to control human population: malnourishment, starvation,
disease, stress and violence." The choice is simple, says
Pimentel-total reproductive freedom now, or freedom from
suffering in the not-so-distant future.

Cornell researchers presented their findings at the
annual meeting of the American Association for the Advancement
of Science in San Francisco. Their results were published in
the May [1994] issue of Population and Environment. Joining Pimentel
as co-authors were Marcia Pimentel, MS, retired senior
lecturer in the Division of Nutritional Sciences, and
Cornell undergraduates Rebecca Harman, Matthew Pacenza and
Jason Pecarsky.

Optimum world population: 2 billion. Optimum U.S.
population: 200 million. Ecologists call these magic numbers
"carrying capacities." An area's carrying capacity (or "K") is
the number of individuals of a given species that can be
supported indefinitely by the local environment. K is defined
by limiting factors-predation, limited space, limited amounts
of food, competition from other species and other threats.

You may remember the ominous graphic representation of
carrying capacity from your introductory biology course.
the number of individuals erupts as the species
exploits an area's supply of food and space, reproducing
unchecked. As resources run out, the population first
plateaus, then plummets sickeningly as individuals die,
eventually stabilizing at a number just below the almighty K.

Where does Pimentel put us on the
graph
of Earth's human population? "Right now, while we still have
relatively abundant fossil fuel energy, I would say that we're getting
near the inflection-the point where the sharp rise in population
begins to flatten off."

Sharp rise, indeed. Almost 5.8 billion people now live on
the planet. The population is projected to reach nearly 8.4
billion by the year 2025 and 11 billion by 2100. Every day the
total rises by a quarter-million. The relative abundance of
young people in their reproductive prime gives the world
population explosion surprising momentum. "Even if we adopt a
zero population growth strategy tomorrow-a little over two
children per couple-the world population will nearly double
[by ?]," explains Pimentel. "It wouldn't stop growing for 60
years."

According to the Cornell team, the primary limiting
factors that define the Earth's carrying capacity for humans
are fertile land, fresh water, fossil fuel energy and a
diversity of helpful natural organisms. All are essential in
the production of food, and worldwide stocks of all four are
being consumed faster than they can be replaced.

More than 25 million acres of arable and pasture land,
the source of 98 percent of the world's food, are critically
degraded and abandoned each year. An additional 12.5 million
acres of new fertile land must be put into production to feed
the 92 million new mouths added each year to the world
population. One and a quarter acres of good crop land are
needed to provide a diverse, nutritious diet of plant and
animal products for one person. The current world average of
crop land per capita is just over two-thirds of an acre.

Fresh water supplies are being overdrawn from surface and
groundwater sources. The primary consumer: agriculture, which
absorbs, without possibility of recovery, 87 percent of the
fresh water used each year in the world. "It takes 1,400
pounds of water to produce one pound of food," says Pimentel.

Fossil energy from oil, gas and coal may be the first
limiting resource to disappear. Essential for industrial
production, fuel, construction, heating and cooling,
packaging, delivery of clean water and fertilizer manufacture --
about 319 quads (that's 10,000,000,000,000,000 BTUs in
air-conditioner-ese) of fossil fuel are used worldwide each year
(one-fourth of which is burned in the United States). The
Cornell team cites research suggesting that world supplies of
oil and gas should last about 35 years at current pumping
rates, coal and uranium stocks about 100 years.

The easiest natural resource to overlook is provided by
the living organisms that share the planet with us. "Humans
have no technologies that can substitute for the services
provided by wild biota," says Pimentel. "There are about half
a million species of animals, plants, and microbes that
provide essential functions for humans in the United States."
Large-scale food production would be impossible without
pollinators, decomposers, scavengers and waste recyclers.
Biodiversity also ensures a future gene bank for tomorrow's
forestry and agriculture. Researchers estimate that 150
species are lost daily due to human activity. (See
"Biodiversity: What's in it for Us?" November 1993, Cornell
Magazine.)

In order to arrive at their optimum population figures,
Pimentel and his colleagues make the optimistic assumption
that humanity will make maximum use of Earth's finite
resources. Even if humans make a transition to renewable
energy sources, stop polluting and degrading their environment
and accept a standard of living equal to one-half of that
enjoyed by Americans today, the numbers stay the same.

Getting there will be painful, Pimentel admits, but
achievable. Population reduction will require a growing
proportion of elderly to be supported by a shrinking number of
younger, more productive humans. "It will create economic and
social stress when we have to makes these changes," he
acknowledges. "But our alternative is far worse economic and
social stress." If global human fertility can be reduced from
the current rate of 3.3 children per female to 1 to 1.5
children per female, Pimentel estimates that the population
can be reduced to carrying capacity in 100 to 150 years.

Population control presents ethical problems. "You have
to take your choice," explains Pimentel. "People say that they
should have the freedom to reproduce. I'm sympathetic to that
view. But you're going to either lose some of that freedom, or
your children and your grandchildren will lose some of their
freedoms-freedom from starvation, freedom from disease."

Pimentel bristles at critics (rumored to include high-
voltage media commentator Rush Limbaugh) who claim that
technology will allow humanity to keep pace with a steep
population curve. "Look at fishery production. We built bigger
ships, larger nets, and now the fish populations of the oceans
lakes and rivers are lower than they've been since 1970. Look
at the Colorado River. As it flows south, California, Arizona,
and Colorado take a big piece out of it to support their
populations. By the time that river reaches Mexico, it's dry.
What technology do we have available, short of manipulating
the climate, that can double the flow of the Colorado River?"

And how does Pimentel respond to those who say that the
endless, painful cycle of population booms and busts is
nature's way? "I think that we're too intelligent to let
nature control our numbers," he says, ultimately the optimist.
"I think that we can do a more effective job of limiting our
numbers than nature can."